There is free tool to merge PST files. If you need to merge multiple PST files, the tool is easier and safer solution when both PST files are opened in Outlook. PST Merger will merge selected PST files automatically into newer PST file without modifying data from original PST files.
Check the free procedure of this tool, here:- Combine Outlook PST files.

Microsoft Outlook gives us free outlook compaction option but it is not safe for PST files because sometimes it misplace email files from Outlook mailbox and does not remove and reduce extra spaces, white spaces, virus affected database in PST files. It Is good for those users who have minimum PST files in Outlook folder. But if you have heavy database and want to really save from damages then you have to choose Compact PST easily get corrupt due to its large size so its quite necessary to compress PST file just before its reaches to threshold limit. You can use in-built compression feature and remove mail attachments as well but this process did not very satisfactory one.